This invention relates to a spark plug for internal combustion engines, and more particulary to a small-sized spark plug having smaller dimensions as a whole than conventional ones.
The spark plugs now in general use have a large outer diameter (the diameter of the threaded portion of spark plugs now in use is generally 14 mm). This fact becomes a serious obstacle to the design of the installation for the water cooling system and valves at the head portion of an engine when it is intended to make smaller-sized and lighter spark plugs or spark plugs capable of multipoint ignition.
FIG. 5 shows the main part of a spark plug which is now in general use. In the spark plug shown in the Figure, gaseous discharge occurs in the air-gap (a) formed by a center electrode 1 and an earthed electrode 2a. If the size of this type of spark plug is reduced in accordance with the object of the invention, the distance (b) between the center electrode 1 and the housing 2 is shortened. It is undesirable, however, for the distance (b) to be made too short, because this would mean creation of a spark passage (d) directly between the center electrode 1 and the housing 2 as well as the a spark passage (c) in the normal spark gap and this would in turn cause a spark between the center electrode 1 and the housing 2 along the surface of the insulator which might result in inferior ignition efficiency, misfiring and fluctuations in combustion.
On the other hand, if the outer diameter of a spark plug is made smaller by reducing both the outer diameter of a center electrode and the thickness of the insulator, and by employing creepage discharge as is described in Japanese Patent Publication No. 1496/1972 in place of the gaseous discharge so that sparking may occur directly from the center electrode to the housing along the insulator surface, several channels are formed on the surface of the insulator due to abrasion by sparks; namely, what is called a "chanelling phenomenon" is produced. When the channels are caused on the insulator surface by virtue of this channelling phenomenon, sparks travel along the channels and the area in which sparks came into contact with a mixture gas is reduced, which leads to deterioration of ignition efficiency. Furthermore as the channelling phenomenon progresses, the channels on the insulator surface between the center electrode and the housing take the shape of bores through which sparks travel so that they have substantially no contact with the mixture gas. This phenomenon thus disadvantageously results in a remarkable reduction in durability of the spark plug as well as failure to cause ignition.